In wet offset printing, problems frequently arise as a result of the difference in the amount of water retained in non-dried webs as compared with the amount of water retained in dried webs, and as a result of the different print materials, and especially as a result of the printing inks. In wet offset printing, the print substrate absorbs water nearly continuously following its initial contact with the dampening agent in the first printing couple. This continues until the layer of water that is applied to the surface of the paper during printing has been completely absorbed and then evaporated. Thus, in wet offset printing, the volume of the print substrate increases nearly continuously, while the modulus of elasticity decreases nearly continuously. As a result, the width of the paper web also increases nearly continuously. In printing, this is manifested as a so-called fan-out effect. The image portion that is printed with the first ink is wider, due to the effect of water on the paper, and the subsequent image segments must be made wider, by the same degree, in order to have the image printed with equal coverage, true to register. The decrease in the modulus of elasticity, which is caused by water absorption, results in increased lengthening of the web of paper, which is under tensile stress, and to a decrease in this tensile stress. For an average newsprint paper, and for a tensile stress level of 20 dAN/m, for example; a linear expansion of the web, of approximately 0.04%, occurs when the web is dry, for example, and a linear expansion of the web, of approximately 0.20%, occurs when the web is wet, after it leaves the printing unit, for example.
During passage through a dryer, the absorbed water in the web is practically completely evaporated, and the drying web of paper shrinks correspondingly, while the modulus of elasticity increases again. In hybrid production in wet offset presses, these effects can lead to problems with web lead and register.
A printing press system for producing coldset/heatset hybrid products is known from WO 2007/020288 A1. Different types of paper and different inks are used for coldset and heatset production.
DE 20 2005 020947 U1 discloses planar blanket-to-blanket printing units with printing couple cylinders that are mounted in linear bearings and which can be adjusted using power controls. For different paper qualities, different levels of pressure, for engaging the print-on setting, are provided.
In the disclosure of WO 03/045694 A1, it is recommended that an ink having a tack of 6 to 9.5, as measured using an Inkomat device manufactured by the Prüfbau company, should be used for dry offset printing in the printing couple under pressure conditions.
DE 10 2006 038 638 A1, WO 2007/071459 A1 and DE 10 2006 038 638 A1 each disclose, among other things, a “heatset printing unit,” which can be operated in one production mode using heatset ink and which can be operated in another production mode using coldset ink. Heatset and coldset inks are defined as inks having different properties and compositions.